You may be in a situation where a null value is not acceptable for a parameter, or there may be situations that require you to restrict the input you receive. While such situations can be handled with branch-based validation, that would mean dedicating some amount of code to such validation. Parameter validation can help here, without you having to worry too much about what construct to use for the validation.
Moreover, parameter validation can also assist with tab completion!
The scenario for this recipe is to limit the value of TotalTime to one of 5, 10, 15 and 20 seconds.
